Plasticized polyvinyl butyral (PVB) sheet is used in the manufacture of laminate structures such as, for example: windshields for vehicles including automobiles, motorcycles, boats and airplanes; homes and buildings; shelving in cabinets and display cases; and other articles where structural strength is desirable in a glass sheet. In many applications, it is desirable that the laminate be transparent and colorless, or at least have very low color. Undesired or unintended color in a glass laminate can be a result of impurities from various sources. In some cases, color can occur in the PVB interlayer. Color in a PVB sheet can result from several sources in the PVB resin, or from the manufacturing process. For example, color can result from instability of the PVB resin, from impurities, or from other additives present in the PVB composition. Color in a PVB sheet can develop during storage of the PVB, or be caused by process conditions to which the resin is subjected.
Conventional PVB sheet typically includes a plasticizer in order to increase the flexibility and processibility of the PVB sheet. Generally, the higher the concentration of plasticizer, the more flexible the sheet. Various plasticizers are conventional in the manufacture of PVB, and include such plasticizers as: diesters of polyethylene glycols such as triethylene glycol di(2-ethylhexanoate) (3GO) and tetraethylene glycol diheptanoate (4G7), for example. These examples are not all-inclusive of known plasticizers useful for preparing PVB sheeting. Typically, plasticizer is included in amounts of greater than 30 parts per hundred (pph), based on the total weight of the resin. Depending upon the application, as well as other factors, highly plasticized PVB typically can have as much as 60 pph of plasticizer.
In a conventional PVB sheet manufacturing process, additives are typically included to protect PVB from developing color, to improve the manufacturing process, or to impart desirable properties or characteristics to the sheet. Examples of such additives are: antioxidants, such as octyl phenol for example; light stabilizers; surfactants; and adhesion control additives.
Manufacturers are continually looking for opportunities to improve the process or the properties of the product. For example, combinations of light stabilizers can be required for optimum performance in conventional PVB compositions. Conventional PVB sheet compositions can include, for example, Tinuvin® P, Tinuvin® 123 in addition to Tinuvin® 326 to obtain satisfactory light stability in the sheet. However, using combinations of light stabilizers can add additional expense and complexity to the manufacturing process, which is not desirable.
One other problem faced by PVB manufacturers is that changing one component or additive in the polymer recipe can affect the performance of other additives, or change the quality or performance of the final sheet. There can be totally unexpected problems or benefits that result from a change in the composition of a polymer recipe. For example, Applicants have discovered that the effectiveness of a particular antioxidant can be dependent upon the plasticizer used. For example, the Applicants surprisingly have found that antioxidants that are effective in combination with 4G7 as plasticizer are not as effective when using 3GO plasticizer.
It can be desirable to have a process for preparing a plasticized PVB sheet composition, whereby the color of the PVB sheet can be improved with the proper selection of additives. It is a further object of the present invention to have a process for manufacturing PVB that utilizes a plasticizer and a compatible antioxidant/additive package.